Method and apparatus for coating vitreous tubes



Oct. 31, 1939.

W M. CLARK El AL METHOD AND APPARATUS FOR COATING VI'IREOUS TUBES FiledDec. 11, 1936 .1 s c n Y w r x 0 *m u m w n S @C a T h Patented Oct. 31,1939 UNITED STATES METHOD AND APPARATUS FOR COATING VI'IREOUS TUBESWilliam M. Clark Cleveland, Ohio,

and Charles D. Spencer, East assignors to General Electric Company, acorporation of New York Application December 11, 1936, Serial No.115,402

7 Claims.

Our invention relates to a method of and apparatus for coating vitreoustubing and more particularly to a method and apparatus for applyingcoatings of an enamel or glaze to the inner surfaces of glass tubesused, for example, as containers or ,envelopes for'electric lamps.

Heretofore it has been the practice to place the coated glass tubes inan oven maintained at the required temperature to fuse the coating.Compared to the method and apparatus comprising our invention, thismethod is very inefficient due to loss of heat from the greater areaexposed and to the greater time required for the heat to soak" throughthe glass tube to the coating. Oven heating is also disadvantageouswhere the heating takes place in an evacuated space or in a space filledwith a gas which facilitates the treatment of the coating due to itslarge size and the likelihood of contaminating the gas.

According to our invention, the internally coated vitreous tubes areheated by means of an elongated heating element, preferably anelectrical resistance element, extending through the interior thereof.The tube is preferably rotated about its longitudinal axis to moreevenly distribute the heat and it may be closed at the ends so that theinterior may be treated by a gas if desired. In this manner the coatingon the inner surface can be brought to a high temperature before thewalls of the tubular body reach the softening point. Where a gaseousatmosphere for treating the coating is to be employed, the gas isintroduced and exhausted through the end closures which also providemeans for supporting the heating unit and conducting the electricalenergy thereto. Other features and advantages of our invention willappear from the following detailed description and from the accompanyingdrawing.

In the drawing, Fig. 1 is a perspective view of apparatus comprising ourinvention, and Fig. 2

I is a sectional view of a modified portion thereof.

Referring to Fig. 1, the glass tube I which is to be treated issupported on facings II of pairs of rollers I2, I3 and I4, I5 which holdsaid tube I0 in a position surrounding a heater element I6 which extendstherethrough. The facings II of the rollers I2, I3, I 4 and I5 arepreferably made from asbestos so as not to unduly chill the glass tubeI0 and so as to engage said tube It! with sufficient friction to causeit to rotate as rollers I2, I3, I l and I5 are rotated. Rollers I2 andI4 are mounted on a shaft I1, and rollers I3 and I5 are mounted on theshaft I8. The shafts l1 and I8 are supported by bars I9 and and arerotated in unison through pulleys 2I which are coupled by a belt 22. Theshafts I 'l and I8 are driven through a pulley 23 on shaft I8 by a belt24 which is connected to a source of power not shown. The rotation ofthe glass tube I 0 causes the heat from the element I 8 to be evenlydistributed circumferentially about the said tube.

The heating element I6 may be a Globar" rod in which the actualelectrical resistance member is enclosed for protection and support.However, other types of heating units which are selfsupporting and havethe electrical terminals therefor at the ends may be used. The endportions of the heater I6 fit into apertures in blocks 25 and 26 andabut against resilient U-shaped metal strips or springs 21 and 28, whichalso serve as electrical connections thereto. The heat from the elementI6 is shielded from the springs 21 and 28 by the blocks 25 and 26respectively, which are preferably made of asbestos and are attached tosaid springs 21 and 28. The electrical energy required for the heatingelement I6 is conducted to the springs 21 and 28 from a source notshown, by cables 29 and 30, and is insulated from the rest of theapparatus by insulating plates 3| and 32, which separate the springs 21and 28 from their respective supporting standards 33 and 34. The bars I9and 20 are also attached to the standards 33 and 34 and may be adjustedvertically thereon to locate the heating element I6 in the desiredposition within the glass tube III.

The glass tube I0 is prevented from shifting longitudinally by a pair ofdiscs 35 and 36 which aresomewhat larger than rollers l2 and I 4 and aremounted on shaft H at points adjacent to the ends of said tube I 0.

Although the specific heating element I6 shown gives off substantially auniform amount of heat from end to end, it may be desirable tocompensate for or prevent the escape of heat from the open ends of theglass tube III in order to treat said tube evenly from end to end. Inorder to compensate for the loss of heat, the heating element I6 ispreferably constructed so as to have a greater heating capacity from thecenter outward toward the ends. To prevent the escape of heat from theopen ends of the tube III, the said ends may be closed off as shown inthe modification in Fig. 2. In the modification, the blocks 25 and 26are both provided with bosses 31 which carry flanged washers 38, whichin turn engage the ends of the glass tube Ill. The washers 38 turn withthe tube I0 and effectively close off the ends thereof. Each of thebosses 31 is also provided with an aperture 39 which permits changingthe atmosphere within the said tube I0. Because of the cooling effect ofthe bosses 31 and washers 38 on the tube construct the heating elementI6 so that the ends are kept at a higher temperature than the center, inorder to bring the said tube III to substantially the same temperaturefrom end to end. The apertures 39 in the bosses 3! permit exhausting theinterior of the tubular body ID or filling the interior with a gas inertto the inner surface, or

III, it may be desirable to a gas capable of combining with or otherwisetreating said inner surface. If a gas is to hemtroduced into theinterior of the tube II, it may be passed in through one pipe 40 andwithdrawn through the other pipe ll.

The glass tube to be coated or enameled may be smooth on the inside ormay be given an etch similar to the etch developed on the inside of acidfrosted incandescent lamp bulbs. Thematerial to be coated on the insideof the tube may be a solid held in suspension in a liquid and sprayed onthe inside surface, or some liquid saturated with a soluble material maybe sprayed on or applied by dipping or flowing on the inside surface.The object is to coat the inside of the tube with a layer of thematerial to be burned on by the application of heat. If a transparentglaze is to be developed, the softening temperature of the coatingmaterial must be less than the softening temperature of the glass tube.An opaque or translucent coating, either colored or colorless, may beapplied also. Such a coating should contain some ingredient ofrelatively low softening point to act as a binder.

The glass tube l0, either smooth or etched, is coated on the inside witha uniform layer of the coating or enameling mixture. The coated tube isplaced on the rotating device and the heating element put in position.The rotating device is started so as to revolve the tube at about 25 R.P. M. and the coating is dried out by slowly heating the glass tube bymeans of a controlled current through the heating element It. When thecoating has been thoroughly baked out at a relatively low temperature,the current through the heating element is gradually increased until thesoftening temperature of the glass tube is approached. The tube is thenmaintained at this relatively high temperature until the coatingmatures. In the case of transparent enamels melting and flowing willoccur. The operation is completed when the enamel layer has assumed asmooth and uniform texture. When an opacifying agent has been includedin the coating mixture the resulting enamel is opaque or translucent. Inthis case the tube is heated at a relatively high temperature until thebinding material has softened sufficiently to hold the coating mixturein intimate contact with the inside surface of the tube.

We have discovered that, by use of the method and the apparatusdescribed above, it is possible to coat a Pyrex tube on the inside witha thick or a thin layer of fused sodium borate. The Pyrex tube is firstetched. A saturated solution of sodium borate in water is thenintroduced. The etch will hold an appreciable quantity of the solutionand the excess is drained off. The tube is then heated according to theprocedure described above.

Another particular application of this method is the coating of anetched Pyrex tube with an enamel of the following composition: 3320s,2Se0z, l/8NaNO3 (molecular proportions). This enamel dissolves in waterand the tube is coated with a saturated solution of this mixture intowhich some very fine ground solid material of the same composition hasbeen introduced. The application of this selenium borate mixture to theinside of an etched "Pyrex tube develops a semitransparent surface onheating. The surface layer seems to be continuous and does not exhibitchecks or cracks.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Apparatus for heating the inner surface of a vitreous tube comprisingan elongated electrical resistance heating element with supporting meanstherefor, means for supporting said tube independently of the heatingelement in a position surrounding said heating element and spacedtherefrom and means for rotating said tube about its longitudinal axis.

2. Apparatus for heating the inner surface of a vitreous tube comprisingan elongated electrical heating elements with supporting means thereforand means for supporting said tube independently of the heating elementin a position surrounding said heating element, said heating elementhaving greater heating effect at the ends than at, the middle.

3. Apparatus for heating the inner surface of a vitreous tube comprisingan elongated electrical heating element with supporting means therefor,

means for supporting said tube independently of the heating element in aposition surrounding said heating element, means for closing of! theends of said tube to retain the heat therein, and means for rotatingsaid tube about its longitudinal axis.

4. Apparatus for heating the inner surface of a vitreous tube comprisingan elongated electrical heating element with supporting means therefor,means for supporting said tube independently of the heating element in aposition surrounding said heating element, means for closing oi! theends of said tube from the atmosphere, said lastnamed means havingapertures therethrough for permitting a gas to be passed through saidtube during the heating thereof, and means for rotating said tube aboutits longitudinal axis.

5. The method of coating a vitreous tube which comprises applying alayer of an enameling mixture to the inner surface of said tube, placingsaid tube around and spaced from an elongated heating element, androtating said tube about its longitudinal axis during the heating ofsaid enameling mixture.

6. The method of coating a vitreous tube which comprises applying alayer of an enameling mixture to the inner surface of said tube, placingsaid tube around and spaced from an elongated heating element andholding it in a substantially horizontal position, and rotating saidtube about its longitudinal axis during the heating of said enamelingmixture.

'7. The method of coating a vitreous tube which comprises applying alayer of an enameling mixture to the inner surface of said tube, placingsaid tube around and spaced from an elongated electrical heatingelement, rotating said tube about its longitudinal axis while passing acontrolled electric current through said heating element to bake saidenameling mixture at a relatively low temperature, gradually increasingthe current through said heating element until the softening temperatureof the vitreous tube is approached and then maintaining the tube at thishigh temperature until the coating matures.

WILLIAM M. CLARK. CHARLES D. SPENCER.

